The present invention is useful primarily with water cooled nuclear reactors. It differs from known "core catcher" systems that are used in reactors for dealing with long term decay heat removal when the cores of liquid metal reactors or gas cooled reactors are disrupted, causing a relatively low pressure or benign-condition downward flow from the reactor of molten core material under the force of gravity. It is also recognized that nuclear reactor design concepts have been proposed which include a variety of structure designs for "in-vessel" core catchers that are normally positioned directly below the nuclear fuel elements of the reactor, or are otherwise situated within the reactor pressure vessel. Unlike such known prior reactor the present invention provides a melt-retention structure that is positioned externally from the reactor pressure vessel of a water-cooled nuclear reactor. This invention is designed to operate under conditions imposed as a result of a reactor core meltdown under high-pressure vessel conditions, followed by failure of the lower vessel boundary. This invention would operate under the very high velocities that may prevail immediately following the initial discharge of molten core material from the reactor vessel if such an accident should occur.